The present invention relates generally to systems for positioning a first object relative to a second object and, more particularly, to an apparatus for precisely positioning a movable platform or stage along an axis of motion.
The need for positioning accuracy of a mechanical stage for both large (coarse) and small (fine) motion has been increasing with the progression of semiconductor manufacturing technology such as microlithography. Lithography involves various techniques for selectively removing or adding material to semiconductor wafers from which, for example, very large scale integrated (VLSI) circuit components may be fabricated.
Masked ion beam lithography, in which a collimated beam of ions passes through a mask onto a semiconductor wafer covered with photoresistive material, provides extremely high pattern resolution. However, because there is some ion scattering when the ions travel through the mask, the mask and wafer must be positioned very closely to each other to achieve high-pattern resolution exposures.
Additionally, once the mask is fixed in location, alignment of the wafer relative to the mask requires very precise motions. The motions may involve four and as many as six degrees of freedom. This movement needs to be accomplished very rapidly, and the wafer needs to be held rigidly in place once it is precisely located.
Step-and-repeat projection alignment and exposure systems are employed both in the fabrication of photomasks and in the processing of semiconductor wafers. A high (sub-micron) resolution photomask may be fabricated by using a precisely-controlled stage movable along coordinate axis of motion. The stage may successively position adjacent regions of the mask relative to an image (formed by a projection lens) of a reticle containing a level of microcircuitry that is to be printed on the photomask. An array of adjacent regions of microcircuitry of one level may be formed on the photomask in rows and columns parallel to the stage's coordinate axes of motion. A set of such masks, each bearing an array of microcircuitry of a different level, can be employed to fabricate integrated circuits from a semiconductor wafer.
The semiconductor wafer may be sequentially aligned with each photomask of the set, and a level of microcircuitry printed on the photomask may be, in turn, printed on the wafer. This can also be done by using a precisely-controlled stage movable along coordinate axis of motion to successively position adjacent regions of the wafer relative to the mask.
Additionally, the mask set fabrication operation may be eliminated; and instead, a precisely-controlled stage may be used to successively position adjacent regions of the wafer relative to each of the reticles employed in fabricating the photomasks. Thus, the level of microcircuitry contained on each those reticles may be printed directly onto the wafer during separate step-and-repeat printing operations.
The above-described operations require accurate measurement of microscopic objects whose sizes are in the range of 1 to 50 micrometers (.mu.m). Displacement of these objects on the stage may be measured by an interferometer, an encoder, or a linear variable differential transformer. The stage must be capable of linearly positioning objects over the required range with resolutions of at least 0.001 .mu.m or less with a motion that is free as possible of vibration, pitch, roll, and yaw.
Micropositioning stages used heretofore have employed mechanical bearings and screws to permit the required movement of an object carried by the stage. Additionally, where movement of the object along two axial directions is required, stages have been stacked one on top of the other in a vertical arrangement. This procedure, while permitting biaxial orientation and displacement of the object, also results in undesirable deviation in accordance with the "Abbe" principle. See J.B. Bryan, "The Abbe Principle Revisited An Updated Interpretation", Lawrence Livermore Laboratory, May 14, 1979.
Other systems used heretofore for wafer lithography have incorporated a rotating stage on top of a translational stage. Translational motion of the wafer in a plane and rotation of the wafer about an axis normal to the plane are allowed. The plane of translational motion is commonly referred to as the x-y plane, and the angle of rotation in the x-y plane as .theta..
It is an object of the present invention to provide a very accurate and precise stage positioning apparatus for movement of an object.
Another object of the present invention is to provide a micropositioning device for movement of an object in two directions which avoids the "Abbe" deviation heretofore encountered in systems using multiple stacked stages.
It is a further object of the present invention to provide an inexpensive stage positioning apparatus for movement of an object over a wide dynamic range.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.